Perioperative ruminal pH changes in domestic sheep (Ovis aries) housed in a biomedical research setting

Abstract

Little information is available on normal ruminal pH values for domestic sheep (Ovis aries) housed in a research setting and fed a complete pelleted ration. Sheep maintained on pelleted diets undergoing surgical procedures often present with postoperative anorexia and rumen atony. To determine the relationship between diet and postoperative rumen acidosis and associated atony, we studied dietary effects on ruminal pH in an ovine surgical model. Sheep undergoing orthopedic surgical procedures were randomized into 2 diet groups. Group 1 (n = 6) was fed complete pelleted diet during the pre- and postoperative period, and group 2 (n = 6) was fed timothy grass hay exclusively throughout the study. Measures included ruminal pH, ruminal motility, and rate of feed refusal, which was monitored throughout the pre- and postoperative periods. The 2 groups did not differ significantly before surgery, and the ruminal parameters remained largely within normal limits. However, a downward trend in the strength and frequency of rumen contractions was observed in pellet-fed sheep. After surgery, the pellet-fed group showed clinical signs consistent with ruminal acidosis, supported by decreased ruminal motility, anorexia, putrid-smelling ruminal material, and death of ruminal protozoa. Intervention by transfaunation in clinically affected sheep resulted in resolution of signs. Our findings suggest that sheep fed grass hay appear to have a more stable ruminal pH, are less likely to experience anorexia and rumen atony, and thereby exhibit fewer postoperative gastrointestinal complications than do sheep on a pellet diet.

Figure 1.

Ruminal fluid samples from sheep fed (left) pellets or (right) grass hay. Ruminal fluid from the pellet-fed sheep is brown in color and has less particulate matter, whereas that from grass-fed sheep is greener in color with more particulate matter.

Figure 2.

Mixed-effects regression model with fixed effects shows the predicted ruminal pH in pellet-fed compared with hay-fed sheep for the 6 wk prior to surgery (weeks −6 to −1), at the time of surgery (day 0), and 7 d after surgery (days 1 to 7).